https://leetcode.cn/problems/contains-duplicate-ii/description/
class Solution {
public:
    bool containsNearbyDuplicate(vector<int>& nums, int k) {
        unordered_map<int,int>hash;
        for(int i=0;i<nums.size();i++)
        {
            if(hash.count(nums[i])&&i-hash[nums[i]]<=k)
                return true;
            else 
                hash[nums[i]]=i;

        }
        return false;
    }
};